The invention is in the field of carbohydrate chemistry, and generally pertains to methods for the preparation of glycosides.
Saccharide-derived glycosides are a commercially important class of carbohydrates. Such glycosides find numerous commercial applications, for example, as surfactants useful in detergents and other products. Conventionally, glycosides are prepared via the acid-catalyzed Fischer glycosylation of carbohydrates. For example, U.S. Pat. Nos. 3,928,318 and 4,797,478 teaches the production of methyl glucoside by glycosylation of glucose with methanol. Another patent, U.S. Pat. No. 4,223,129,teaches the production of alkyl polyglucosides from starch starting materials; other such reactions are taught in U.S. Pat. Nos. 3,219,656 and 4,797,478 and in Hill et all, Alkyl Polyglucosides G. Stoll, ed., VCH Publishers, Weinheim, Germany (1997).
Glycosides produced by the glycosylation of glucose typically are limited to an average degree of polymerization (DP) of about 1.3. Thus, the glucose is unable to form glycosides with higher degrees of polymerization. It has been found difficult to produce alkyl glycosides having a higher DP from higher DP starting materials. As reported in the aforementioned U.S. Pat. No. 4,223,129, for instance, the reaction with starch and an alcohol also results in products that are primarily limited to low DP alkyl glucosides, rather than glycosides having a higher DP as might be expected. If the starch-alcohol reaction is terminated prior to completion, granular starch is obtained, not glycosides having an intermediate DP profile.
The aforementioned prior references do not provide a satisfactory method for producing alkyl glycosides with a higher DP than about 1.3. It is a general object of the invention to provide such a method.
The invention is premised on the surprising discovery that higher alkyl glycosides can be prepared readily and in good yield by glycosylating a malto-oligosaccharide or mixture of malto-oligosaccharides. Although malto-oligosaccharides are similar in structure to starch, but of lower molecular weight, the malto-oligosaccharides do not degrade into glucosides upon glycosylation, as occurs with starch.
In accordance with the embodiment of the invention, a method for preparing a malto-oligosaccharide derived glycoside is provided. Generally, the method comprises providing a malto-oligosaccharide, and glycosylating the malto-oligosaccharide with an alcohol or a thiol under conditions suitable to form a malto-oligosaccharide derived glycoside. In accordance with another embodiment, the invention provides a method for preparing a mixture of malto-oligosaccharide derived glycosides by providing a mixture of malto-oligosaccharides, and glycosylating the malto-oligosaccharides with an alcohol or a thiol under substantially anhydrous conditions to form a mixture of malto-oligosaccharide derived glycosides. Additionally, the invention provides a mixture of malto-oligosaccharide derived glycosides. Surprisingly, the glycosides and glycoside mixtures of the invention are substantially soluble, even those having a high DP.
The conditions for the glycosylation reaction, such as time, pressure, and temperature, may be selected based upon the desired DP weighting of the glycoside product in the reaction (i.e., whether the DP profile of the glycoside mixture is weighted towards lower or higher DP species, relative to other mixtures formed by reacting the same or similar starting materials under different conditions). Generally, although it often is not commercially practicable to specify the glycoside DP profile with exact precision, a desired DP weighting can be selected based upon known reaction parameters. In accordance with one embodiment of the invention, glycosylation reaction conditions for an alcohol or thiol and a mixture of malto-oligosaccharides are selected based on a desired glycoside DP profile. The reaction conditions and their affect on the DP weighting for a given mixture of malto-oligosaccharides preferably are determined empirically. For instance, a glycosylation reaction may be performed under a first set of reaction conditions, and the glycoside DP weighting then may be determined. If the glycoside DP profile is not what is desired (for example, if the profile is weighted too much towards lower DP glycosides), one or more reaction conditions then may be adjusted and the reaction performed under the new conditions. These steps may be repeated using various modifications to the reaction conditions until a glycoside mixture with the desired DP weighting is obtained.
Other features of preferred embodiments of the invention are set forth below and in the appended claims.